Q+A: Study shows trees accumulate more carbon as they age

A new study helps us understand the dynamics of old-growth stands of trees and large trees in forest stands, says Louis Verchot, director of forests and environment research at the Center for International Forestry Research. Photo credit: CIFOR/Aulia Erlangga

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Rather than slowing down with age, trees grow faster as they get larger, new research shows — which means they also absorb more carbon from the atmosphere, a finding that could have implications for the management of forests with old-growth stands.

A paper, Rate of tree carbon accumulation increases continuously with tree size, published earlier this month in the journal Nature, found that a single older, larger tree can absorb as much carbon in a year as is contained in an entire mid-sized tree. While the report’s findings might seem counterintuitive, they are not entirely surprising, says Louis Verchot, director of forests and environment research at the Center for International Forestry Research (CIFOR).

In this Q&A, Verchot — who was not involved with the study — talks briefly about the report’s implications and the further questions it raises.

Q: What is the significance of the study’s findings?

A: The study helps us understand the dynamics of old-growth stands of trees and large trees in forest stands. We have known for quite some time that tropical forests and regrowing temperate forests are taking up considerable amounts of carbon. We have also known that many old-growth tropical forests continue to remove carbon from the atmosphere, and this was clearly demonstrated in the Large-Scale Biosphere-Atmosphere Experiment in the Amazon in the early 2000s. This study helps clarify some of the questions around the mechanisms by which these ecosystems continue to accumulate carbon.

Q: Why has the speed of carbon absorption by older trees been underestimated in the past?

A: Studies going back to the 1990s have shown that large trees continue to increase in girth in old-growth stands. The study puts some perspective on relative growth within stands and helps quantify what this growth means in terms of biomass relative to smaller trees in these stands.

Q: Are the findings of the study surprising?

A. I would not call the results surprising. Like many good scientific studies, this one raises as many questions as it answers. For example, the study is based on the assumption that the logarithmic relationship between biomass and diameter is maintained in very large trees. These trees are typically under-represented when biomass equations are constructed, and this is the case for the equations used here. These equations are made to represent “average” conditions at the scale of a tree stand. Application of such biomass equations to very large trees and in time series would not detect things like progression of rot or senescence and loss of large branches, for example. So hopefully this study will spur more research into stand dynamics and the importance of very large trees in these forests.

Q: What does this mean for models of climate change?

A: Very little, actually. Such models work at a much larger scale than individual trees and are concerned more about the net ecosystem exchange of carbon between the biosphere and the atmosphere. This study tells us about how this exchange takes place, but it does not alter our assessment of the magnitude of this exchange.

Q: What are the implications for forest management?

A: There have been several studies recently on the value of very large trees in ecosystem processes. This paper adds to this evidence and suggests that carbon storage may be one additional ecological service of these trees. Thus, it should strengthen the case for including these concerns in forest management plans.

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